1. Field of the Invention
This invention relates to electrochemical sensors for measuring concentration of chemicals in a solution, and more particularly, to an encapsulated electrochemical sensor and a method for encapsulating such a sensor.
2. Description of Related Art
A number of electrochemical sensors are well known in the art. For example, a potassium ion responsive electrode is described in an article by Pioda et al. entitled "Highly Selective Potassium Ion Responsive Liquid-Membrane Electrode", Analytical Letters, vol. 2, pp. 665-674 (1969). Such a sensor is also disclosed in U.S. Pat. No. 3,562,129. In addition, U.S. Pat. No. 3,706,649 (the '649 patent) teaches a substantially solid ion sensitive barrier containing a macrocytic compound, mineral oil, and a lipid which may be used as an ion sensor. Furthermore, a U.K. Patent Application Serial No. 2,025,629 A, published Jan. 23, 1980, discloses an ion selective electrode assembly. Still further, ion-sensitive field-effect transistor (ISFET) sensors are also disclosed in U.S. Pat. No. 4,020,830 to Johnson, et al.
In many solid state sensors, such as ISFET sensors, it is necessary to allow one region of the sensor to come into contact with a solution to be measured, while other regions of the sensor must remain isolated from the solution. This presents a problem in the manufacture of such sensors. One solution to this problem is disclosed in U.S. Pat. No. 4,233,136, issued to Spaziani, et al.
According to Spaziani, a housing having a cover defining a closed interior chamber is fabricated from a rigid plastic material. An interior tubing having a central tubular passage extends axially between firings to create a path for fluid to flow through the assembly. A portion of the wall of the tubing is interrupted by an opening on one side and contains a membrane chemically bonded to, and integral with, the tube. The inner surface of the membrane presents a linear, smooth, continuous surface defining a linear flow path for sample fluids. This configuration allows the solution to contact the membrane in a controlled manner. However, this method is relatively expensive and labor intensive. Furthermore, this method does not easily lend itself to use with ISFETs and other electrochemical sensing devices which do not employ a membrane.
Another method for selectively bringing a solution into contact with the active sensing area of a sensor while confining the solution is presented in an article entitled "A Flow-Through Cell For Use With An Enzyme-Modified Field Effect Transistor Without Polymeric Encapsulation and Wire Bonding" by Satoru Shiono, et al., Analytica Chimica Acta, 202 (1987) 131-140. The article discloses a silicone rubber sheet laid over a silicone substrate on which FETs have been fabricated. The substrate is mounted on a block. Gold lines deposited on the rubber sheet make contact with pads of the FET. A rectangular hole through the silicone rubber sheet allows the solution to contact ion-sensitive portions of the FETs. The substrate is pressed into contact with the silicone rubber sheet. This structure allows a solution to flow past the ion sensitive portions of the FETs without contacting any other portions of the FETs. However, fabricating and assembling the structure disclosed by Shiono is labor intensive, expensive, and has problems with providing good electrical contact and with fluid leakage resulting in poor yield.
Other methods for encapsulating FETs such that a solution to be analyzed is constrained to contact only an active sensing area of the sensor include (1) etching a flow cell, or flow channel, in a rigid epoxy which encapsulates the sensor, and (2) machining flow cells and then manually mounting the sensors in the flow cells. However, both of these methods are time consuming and expensive.
Therefore, there is need for an inexpensive method for fabricating electrochemical sensor assemblies which allow a solution to come into contact only with an active sensing area of sensors while at the same time preventing the fluid from shorting out the sensor electronics. Additionally, in some cases, a reference electrode is required to be in contact with the solution in close proximity to the sensor to maintain a reference voltage. The present invention provides such an inexpensive method and a resulting sensor assembly.